Recent developments in deep sea exploration technology have expanded the ability to investigate ocean floors and deep sea beds as part of the initial phase of the construction of large marine production facilities and pipelines. Examination of the ocean bed's surface is important in evaluating locations for foundations that will anchor platforms for such things as deep sea oil exploration and production rigs and other structures for mining of the ocean floor. For example, the rise in oil prices world wide has led to the investigation of large scale oil drilling platforms in deeper waters such as the Gulf of Mexico, off the coast of South America, Africa and China. Determining the physical and mechanical properties of the soil is critical for evaluating possible foundation locations (along with currents and other local conditions), and thus physical testing of the ocean floor in deep oceanic waters is of vital importance. The high cost of these structures dictates very precise measurements of the soil conditions, which can be in waters several thousands of meters deep. The most common type of testing performed in these conditions is in situ examination of the mechanical response of the soil using a probe such as a cone penetration test (CPT). Cone penetration tests are widely used for an extensive range of applications from terrestrial soil to shallow marine soil to deep sea soil. A brief description of a typical CPT probe and test can be found at http://www.conepenetration.com/online-book/cf-cone/cf-cone-cone-penetration-test/.
The cone penetration test involves a cone tip and cylindrical body that is forced into the soil at a constant rate. A sensor is coupled to the cone tip to measure the strength of the soil, and the cylindrical body is typically equipped with a sleeve to measure the shear forces on the probe as it slides into and through the soil. While the probe penetrates the soil, continuous measurements are taken of the resistance to the cone's penetration and the frictional forces acting on the surface of the sleeve. Stress gauges located in the cavity of the probe measure the compressive force on the cone due to the resistance of the soil as well as the frictional forces on the outer surface. Cables or wires located in the cavity transmit signals to the surface or recording device where they can be analyzed. The use of cone penetration tests, and piezocone test data, are well known in the art for measuring sub-surface conditions of soil both on land and at sea, and for purposes of brevity a more detailed explanation of the structure and techniques of cone penetration testing is omitted herein.
One problem that is fairly unique to the process of deep sea testing concerns the high pressure that is present in that environment and its effect on the precision of the measurements of the soil strength. Cone penetrometers utilize a Wheatstone bridge type stress gauge to measure the stress on the cone tip due to its contact with the surface it is measuring. However, the large pressures on the cone's exterior due to the hydrostatic pressure from hundreds or even thousands of cubic meters of seawater compress the cone and result in detected loads much greater than those caused by the soft deep sediments. When the measurements occur at depths over a thousand meters, the hydrostatic pressure can dominate the stresses that are the subject of the testing, namely the insertion of the cone into the often soft soil at the ocean floor.
To mitigate the external hydrostatic pressure on the probe, it is known in the art to internally pressurize the probe with a non-conducting fluid such as an oil to balance the probe's internal pressure with the external pressure. The internal pressure can be linked to the external pressure so as to maintain equilibrium in the probe during its decent and at the location of the testing. By filling the internal cavity of the probe with a non-conductive oil the external hydrostatic pressure can be balanced. However, this pressure balancing of the probe creates a Poisson's effect where a longitudinal strain and transverse strain in the load cell are unequal. This longitudinal strain is in the same direction as the compression measured by the probe's contact with the ocean bed, and proportional to the pressurization. Therefore, this introduced stain is many times greater than the strain to be measured.
This Poisson's effect leads to a condition where the strain gauges must be zeroed out prior to the test, or somehow compensated to eliminate this artificial strain that is due to the hydrostatic pressure, prior to measuring the actual strain on the probe due to the resistance of the soil to be measured. Zeroing out the gauges is not an optimum solution, however, because the accuracy of the strain gauges are a function of the maximum load, typically around ±0.1% of the maximum load. In the case of Wheatstone bridge type stain gauges, the maximum load is on the order of 40,000-50,000 KPa corresponding to the hydrostatic pressure, whereas the pressure due to the sea bed soil insertion may be only 10-20 KPa. Accordingly, when dealing with soft soils, the error due to the zeroing can be great compared with the actual measurement of the soil, significantly limiting the precision of the results. The foundational design for deep sea structures is dependent upon the results of such testing, and the cost of these structures dictate that precise soil strength measurements are critical to the success of such multi-million dollar projects. Accordingly, there is an urgent and unfulfilled need to improve the quality and precision of deep sea testing of ocean beds that does not subject the gauges to the enormous hydrostatic pressures that are inherent in in situ testing.